Foam material is a typical micro-scale crushable material due to its special pore microstructures. Its pore distribution is a key factor to its physical–mechanical properties. After the complete collapse of all collapsible pores, its mechanical behaviors are like those of the intact materials. Therefore, the deformation of a foam material has two components: pore collapse and elastoplastic deformation of its intact material. In this paper, a rod model is proposed to quantitatively express the relationship between stress and pore collapse. The rod length, which expresses the pore size in foam materials, is assumed to follow the Weibull distribution. The joint stiffness between rods, which resists bending moment, can change with linking substances and work environments. Hence, a constitutive relation for the crushable component of microstructures is derived. This constitutive relation is then applied to some experimental data of hydrostatic compression to observe its performance.